The present invention relates to a method of preventing early degradation of an exhaust treatment device installed in a diesel powered vehicle.
Exhaust treatment devices such as catalytic converters, including NOx traps and particulate traps, typically comprise an active element, constructed in a suitable manner to perform its desired function, arranged within a metal outer casing. The active element in the case of a catalytic converter may for example comprise a ceramic honeycomb (also referred to as a matrix or a brick) the surface of which includes particles of a catalyst such as platinum. Surrounding the honeycomb is a fibrous mat that acts as packaging to locate the honeycomb within its metal casing.
During manufacture of the exhaust treatment device, the mat is wrapped around the honeycomb before it is inserted into the casing. In operation of the engine, the exhaust treatment device is heated by the exhaust gases and by any exothermic catalytic reaction that takes place within the device and the resultant heating of the mat is relied upon to cause the mat to pop. The term xe2x80x9cpoppingxe2x80x9d refers to the expansion and stiffening of the mat, which serve to locate the honeycomb securely within its metal casing.
The temperature needed to cause the mat to pop, which is approximately 300xc2x0 C., is attained during normal engine operation of spark ignition engines and early degradation of the exhaust treatment devices in such an engine is relatively uncommon.
The inventors herein have recognized that with diesel engines on the other hand, the exhaust gases are not as hot as with spark ignition engines. As a result, the exhaust treatment device often does not reach a sufficiently high temperature for the mat to pop naturally. Consequently, the vehicle may be driven with the fragile active element located only loosely within its casing and this can damage the active element. In particular, the active element will rattle in its casing and exhaust gases may escape untreated by passing around the active element. It is also possible for debris to block the active element causing loss of power.
According to the present invention, there is provided a method of preventing early degradation of an exhaust treatment device installed in a diesel powered vehicle, the treatment device having an active element located within a casing by means of a mat surrounding the active element, which method comprises temporarily operating the engine at an early stage following the installation of the exhaust treatment device in an alternative mode wherein the temperature of the exhaust treatment device is raised sufficiently to cause the mat surrounding the active element of the exhaust treatment device to pop.
It is possible to operate a diesel engine in a mode that raises the temperature of the exhaust system, but such a mode is unsuitable for normal vehicle operation. Such an alternative operating mode has been proposed, for example, for the purpose of regenerating a particulate trap by burning off the trapped soot particles. The present invention proposes running the engine in such a mode at an early stage in the life of the exhaust system to ensure that the mat holding the active element in place reaches the necessary temperature for it to pop and for it to hold the active element securely in place.
The mode of operating the engine to raise the temperature in the exhaust system may comprise varying any one or more of the air supply, injection quantity, injection timing and valve timing.
Preferably, during the alternative operating mode of the diesel engine, fuel is injected into the engine late in the power stroke to ensure the presence of unburned fuel in the exhaust gases. Such fuel will react with the surplus air that is always present in the exhaust gases of a diesel engine to raise the temperature of the exhaust system. Of course, such a mode of operation is undesirable under normal circumstance, as it is wasteful of fuel and increases the HC content of the exhaust feed gases. Nevertheless, the engine can be operated in such a mode temporarily without seriously affecting the vehicle driveability whenever it is desired to raise the temperature in the exhaust system.
It is further possible to throttle the air supply to the engine during the alternative mode of operation. Diesel engines normally operate with wide-open throttle and the load is varied by regulating the supply of fuel. If, however, the intake air is throttled, it has the effects of increasing the pumping losses of the engine to generate more heat and of reducing the mass air flow through the exhaust system to reduce the cooling effect of the surplus air.
For the purpose of ensuring the popping of the mat of an exhaust treatment device, the alternative mode of operation need only be brought into effect once in the lifetime of the device. Such mode switching may be effected either manually or automatically. If effected manually, it can be carried out as part of the vehicle manufacturing process or pre-delivery inspection. A command may be given to the engine electronic control unit to operate in the alternative mode, if such a mode is programmed into the ECU, or the ECU control may be overridden temporarily by an external control unit connected to the engine during vehicle assembly.
For the alternative mode to be triggered automatically, it may be programmed into the engine""s ECU and the latter may be additionally be programmed to operate in the alternative mode when a predetermined condition is detected for the first time, such as when a certain mileage is exceeded by the vehicle or the engine exceeds a given load. Once the alternative mode has been activated for sufficient time to cause the mat of the exhaust treatment device to pop, a flag may be set in the memory of ECU to prevent the engine from running in this mode again for the same purpose. Of course, the alternative mode may be instigated for other reasons by the ECU, such as to regenerate a particulate trap.
It is an important advantage of the invention that it may be implemented inexpensively in software in that popping of the mat in the exhaust treatment device can be ensured by simply reprogramming of the engine""s ECU or by reprogramming of equipment used in the vehicle assembly line.